1. Field of the Invention
The present invention is directed to a method for the production of high purity ammonia. The method advantageously produces large volumes of hydrogen gas at a low cost as compared to prior art methods thus enabling the production of ammonia at a low cost. The ammonia can be used in a number of diverse applications such as for the manufacture of urea for fertilizers.
2. Description of Related Art
Ammonia (NH3) is useful in a number of applications. For example, ammonia is useful in fertilizers, either as ammonia or in the form of compounds such as ammonium nitrate. Ammonia is also useful in a number of chemical processes such as the manufacture of nitric acid, urethane and other compounds. Ammonia is typically manufactured from synthesis gas obtained by steam reformation or partial combustion of natural gas or from the action of steam on hot coke. After removal of the carbon oxides, the gas composition is adjusted to a molar ratio of 3 parts H2 to 1 part N2 and is passed over a catalyst at a pressure of about 300 atmospheres and temperature of about 500xc2x0 C. The most significant cost associated with the manufacture of ammonia is the high cost of producing a pure hydrogen gas stream to be reacted with the nitrogen.
It is known that hydrogen gas (H2) can be produced from many different feedstocks such as natural gas, biomass or water using a number of different techniques such as reformation, gasification or electrolysis. The most common methods are steam methane reformation, coal gasification, non-catalytic partial oxidation, biomass gasification and pyrolysis, and electrolysis.
Steam methane reformation is believed to be the most economical and commercially viable process that is presently available. The feedstock is typically natural gas and the feedstock cost represents about 52% to 68% of the total cost. The process forms a gas stream that includes H2 and CO and the CO must be separated from the gas stream to form pure H2.
Hydrogen production from coal gasification is another established commercial technology, but is only economically competitive where natural gas prohibitively expensive. In the coal gasification process, steam and oxygen are utilized in the coal gasifier to produce a hydrogen-rich gas. High purity hydrogen can then be extracted from the synthesis gas by a water-gas shift reaction. Other gases such as fuel gases and acid gases must also be separated from the hydrogen. Hydrogen can be similarly formed by the gasification of hydrocarbons such as residual oil.
The manufacture of hydrogen by steam oxidation is also known. For example, U.S. Pat. No. 4,343,624 by Belke et al. discloses a 3-stage hydrogen production method and apparatus utilizing a steam oxidation process. In the first stage, a low BTU gas containing H2 and CO is formed from a feedstock such as coal. The low BTU gas is then reacted in a second stage with ferric oxide (Fe3O4) to form iron (Fe), carbon dioxide (CO2) and steam (H2O) in accordance with the reaction:
Fe3O4+2H2+2COxe2x86x923Fe+2CO2+2H2O 
The steam and iron are then reacted in a third stage to form hydrogen gas by the reaction:
3Fe+4 H2Oxe2x86x92Fe3O4+4H2 
It is disclosed that the iron oxide can be returned to the second stage for use in the iron oxide reduction reaction, such as by continuously returning the iron oxide to the second stage reactor via a feed conduit. At least one of the stages takes place in a rotating fluidized bed reactor.
U.S. Pat. No. 4,555,249 by Leas discloses a gas fractionating unit that contains a reagent powder, such as an iron alloy, having a significant weight difference between the reduced form and the oxidized form. The unit includes an oxidation zone and a reduction zone for containing the reagent powder wherein hydrogen gas is extracted from the oxidation zone. As the reagent powder is converted from the oxidized to the reduced form, the weight of the powder increases and the change in weight is utilized to transfer the reduced powder to the oxidation zone while moving the oxidized powder to the reduction zone.
The article xe2x80x9cH2 from Biosyngas via Iron Reduction and Oxidationxe2x80x9d, by Straus et al., discloses a method for hydrogen production from biosyngas. The biosyngas, which included H2, CO, H2O, and CO2 with traces of N2 and CH4, was used to reduce magnetite (Fe3O4) to iron (Fe). The iron was then cooled and fed to a hydrogen gas generator where the iron was contacted with steam to form hydrogen by steam-oxidation. The iron oxide was then cooled and returned to the reduction reactor for reaction with the biosyngas.
Other metal/metal oxide systems have been used in addition to iron/iron oxide. For example, U.S. Pat. No. 3,821,362 by Spacil illustrates the use of Sn/SnO2 to form hydrogen. Molten tin is atomized and contacted with steam to form SnO2 and hydrogen gas. The SnO2 is then contacted with a producer gas composed of H2, N2 and CO, which is formed by contacting powdered coal with air. The SnO2 is reduced to liquid tin, which is then transferred back to the first reactor. A similar method is illustrated in U.S. Pat. No. 3,979,505.
There remains a need for an economical process for the production of ammonia from nitrogen and hydrogen. It is believed that the primary hindrance to the economical production of ammonia and related products is the high cost of the hydrogen reactant.
The present invention is directed to a method for the economical production of ammonia having a high purity.